1. Field of the Invention
The present invention relates to a composite sensor device having an angular velocity sensor and an acceleration sensor, and a method of producing the same.
2. Description of the Related Art
In some cases, an angular velocity sensor and an acceleration sensor, paired up with each other, are used. Considering such cases, a composite sensor device in which the angular velocity sensor and the acceleration sensor are integrated with each other was proposed. For example, Japanese Unexamined Patent Application Publication Nos. 10-10148 and 10-239347 disclose examples of the above composite sensor device, respectively. In the composite sensor device described in these Unexamined Patent Application Publications, the angular velocity sensor and the acceleration sensor are provided on the same substrate. Thus, advantageously, the device can be inexpensively produced and miniaturization can be easily achieved.
However, the composite sensor devices proposed as described above have the problems that the sensitivities for detection of an acceleration are undesirable. The reason is as follows.
The vibration type angular velocity sensor contains a vibrator. If an angular velocity is added while the vibrator is driven to vibrate at a constant frequency, a Coriolis force is generated. Due to the Coriolis force, the vibrator is displaced. The angular velocity is detected by sensing the displacement caused by the Coriolis force. The higher the velocity of the above driving vibration is, the larger the Coriolis force is. To improve the detection sensitivity of the angular velocity sensor, it is necessary to vibrate the vibrator of the angular velocity sensor at a high frequency of several kHz and moreover at a large amplitude of several mm for example.
However, the fine vibrator formed in a micro-machining technique is significantly influenced with damping caused by air. The damping force is generated in proportion to the velocity of the vibrator. Accordingly, if the vibrator is vibrated in the atmosphere at a high speed, namely, at the above-mentioned high frequency, and at a large amplitude, the damping exerts a large influence, so that the vibrator can not be vibrated at the above-mentioned high frequency and large amplitude. As a result, the detection sensitivity of the angular velocity sensor is remarkably deteriorated.
Accordingly, the following may be proposed. A lid is attached to a substrate having the angular velocity sensor and the acceleration sensor in such a manner as to cover, at a distance, the upper side the vibrator of the angular velocity sensor and a movable member constituting the acceleration sensor. Thus, the vibrator of the angular velocity sensor and the movable member of the acceleration sensor are accommodated in the same space defined by the lid and the substrate. The space is hermetically sealed in the vacuum state (reduced pressure state) in which the vibrator can be vibrated at a high frequency and at a set amplitude or higher sufficiently for detection of an angular velocity. Thereby, the vibrator of the angular velocity sensor can be vibrated at a desired high frequency and at a sufficiently large amplitude for detection of an angular velocity.
However, the movable body of the acceleration sensor, disposed in the vacuum space in which the vibrator of the angular velocity sensor is also set, is ready to be vibrated. When vibration of the vibrator of the angular velocity sensor is transmitted to the movable member of the acceleration sensor via the substrate, the movable member of the acceleration sensor is vibrated at a high frequency and a large amplitude similar to those of the vibrator of the angular velocity sensor. It is desirable that the movable member of the acceleration sensor is in the still state while no acceleration is applied. However, if the movable member of the acceleration sensor is vibrated at a high frequency and a large amplitude due to the transmitted vibration of the vibrator of the angular velocity sensor, as described above, the vibration at the high frequency becomes noisy, causing the problem that the acceleration can not be detected at a high sensitivity.
Moreover, if an acceleration having a resonance frequency component with respect to the movable member of the acceleration sensor is applied while the movable member of the acceleration sensor is disposed in the vacuum space as described above, the movable member resonates so that the amplitude is increased, irrespective of the strength of the acceleration. Thus, there arises the problem that the acceleration can not be accurately detected.